Fences.
Grounded wire fences are a source of distortion for data
originating from electrode stations located nearby.

Fences using steel posts that have been driven into the ground are obviously grounded. Other fences may be grounded periodically where wire or gate supports contact the ground, intentionally or due to neglect or disrepair. In either case, field measurements originating from current injection located too close to such a grounded circuit can be substantially distorted.
Avoiding an occasional fence is not difficult. Grounded fences can become a serious problem in intensively-farmed areas or industrial installations.
When the multiple occurrences of grounded fencing becomes a major issue with conventional surveys, 3D E-SCAN remains fully functional due to the high density and multi-directional nature of its field data set. E-SCAN's 3D raw data set supports objective determination of which data have been affected by fences, and which have not. Even subtly-distorted data that may not be obvious to a testing protocol will have minimal effect on 3D inversion, their error-component being averaged down by the overwhelming number of overlapping data from non-compromised electrode pairs. When errant data are eliminated, large numbers of confirmed-valid data fill the gaps and continue to present a fully-competent 3D raw data set.
Conventional sparse-data results have no such advantages, and either suffer the bad data or experience significant gaps in data coverage. An inappropriately-located fence has the potential to spoil an entire conventional survey's results.
Above, the mature-industrial site of the Giant Yellowknife 3D E-SCAN survey is a good example of a high density of predictable and unpredictable grounded fence impediments, overcome by E-SCAN's sheer volume of >self-cleaning raw data measurements.

At right, grounded fences, including electric fences with ground-return circuits, were "everywhere" in parts of the intensively-farmed New Zealand
countryside which was surveyed with 3D E-SCAN in 2007 and 2008 epithermal gold programs.